In a video mixer, two video signals V.sub.1 and V.sub.2 (FIG. 1) are multiplied by a key signal K.sub.1 (having a range of values from 0 to 1) and its complement (1-K.sub.1) respectively, and the two signals V.sub.1 K.sub.1 and V.sub.2 (1-K.sub.1) are additively combined in a summer to produce a composite output signal V.sub.q having the form V.sub.1 K.sub.1 +V.sub.2 (1-K.sub.1). When the key signal K.sub.1 is zero, the input signal V.sub.1 makes no contribution to the signal V.sub.q, regardless of the value of V.sub.1. Similarly, if K.sub.1 is one, the signal V.sub.2 makes no contribution to the signal V.sub.q. The proportion of the signal V.sub.q that is contributed by V.sub.1 determines the opacity with which the scene represented by the signal V.sub.1 is perceived in the composite picture. If K.sub.1 is one, i.e., V.sub.1 represents 100% of the signal V.sub.q, then the V.sub.1 scene (the scene represented by the signal V.sub.1) completely obscures the V.sub.2 scene, regardless of the value of V.sub.2. As K.sub.1 decreases, the extent to which the V.sub.2 scene is obscured in the composite picture is reduced until, when K.sub.1 reaches zero, the V.sub.2 scene is opaque and completely obscures the V.sub.1 scene. Thus, the coefficients K.sub.1 and (1-K.sub.1) determine the relative opacity of the two component scenes: if the coefficient K.sub.1 is greater than (1-K.sub.1), then the V.sub.1 scene at least partially obscures the V.sub.2 scene and appears, to a viewer of the composite scene, to be in front of the V.sub.2 scene.
The multiplication of the signals V.sub.1 and V.sub.2 by the key signal K.sub.1 and its complement (1-K.sub.1) is shown in FIG. 1, in which it is assumed that all signals have five discrete values in the range from zero to unity and have sharp transitions between levels. It will, of course, be appreciated that FIG. 1 is in fact very much simplified, and that in the case of analog signals the range of possible values is continuous, and that transitions for either analog or digital signals would have a finite slew rate.
A video signal V.sub.1 ' is said to be a "shaped" video signal when it is the multiplication product of an unshaped video signal V.sub.1 and an associated key signal K.sub.1. In general, there is no necessary relationship between the video signal and its associated key signal. A production switcher normally receives unshaped video signals and their associated key signals and provides a full screen video signal at its output. No key output is produced.
Shaping has two aspects, namely spatial or X-Y shaping (only the X-dimension is shown in FIG. 1), which determines the area of the composite picture to which the component signal makes a contribution (when K.sub.1 =0, the signal V.sub.1 makes no contribution to the signal V.sub.q), and opacity or Z shaping, which determines, for K.sub.1 greater than zero, the magnitude of the contribution that is made by the component signal to the composite signal V.sub.q. The shaping of the component signals is discussed in terms of "coverage" in Porter, T. and Duff, T., "Compositing Digital Images", Computer Graphics, Vol. 18, No. 3 (1984), pages 253 to 259.
The foregoing discussion of the manner of production of the signal V.sub.q is based on the assumption that the signal V.sub.2 is a full field signal, i.e. that the key signal K.sub.2 associated with the video signal V.sub.2 is one for all locations. In the general case, K.sub.2 is not one for all locations and EQU V.sub.q =V.sub.1 K.sub.1 +V.sub.2 K.sub.2 (1-K.sub.1)
It will be seen from this more general expression that the video signal V.sub.q, being the weighted sum of two shaped video signals V.sub.1 K.sub.1 and V.sub.2 K.sub.2, is itself a shaped video signal. For the sake of consistency in notation, the shaped signal that has previously been designated V.sub.q will hereafter be designated V.sub.q ', and V.sub.q will hereafter be used to designate the corresponding unshaped signal.
The key signal K.sub.q that relates V.sub.q ' to V.sub.q is given by EQU K.sub.q =1-(1-K.sub.1)(1-K.sub.2)
If, for every location, either K.sub.1 or K.sub.2 is one, then K.sub.q =1 for all locations. In particular, if either V.sub.1 or V.sub.2 is a full field signal, the signal V.sub.q ' is a full field signal. If, on the other hand, V.sub.q ' is not a full field signal it might be desired to form a composite scene from the scenes represented by the signal V.sub.q ' and, e.g., a background scene represented by a signal V.sub.r having an associated key signal K.sub.r. In such a case, the signal (1-K.sub.q) would be used to process the signal V.sub.r in a production switcher, and an output signal V.sub.s '=V.sub.q '+V.sub.r K.sub.r (1-K.sub.q) would be produced. Generally, V.sub.r would be a full field signal and so K.sub.r =1 and V.sub.s '=V.sub.q '+V.sub.r (1-K.sub.q).
Recalling that V.sub.q '=V.sub.1 K.sub.1 +V.sub.2 K.sub.2 (1-K.sub.1), if K.sub.1 =1, then V.sub.q '=V.sub.1, i.e. the signal V.sub.2 makes no contribution to the signal V.sub.q ', regardless of the value of K.sub.2. Therefore, combining of the video signals V.sub.1 and V.sub.2 is under the primary control of the key signal K.sub.1. Similarly, if the signal V.sub.q ' were equal to V.sub.1 K.sub.1 (1-K.sub.2)+V.sub.2 K.sub.2, the combining would be under the primary control of the signal K.sub.2, and if K.sub.2 =1, then V.sub.q '=V.sub.2 and V.sub.1 makes no contribution regardless of the value of K.sub.1. The two different situations are equivalent respectively to the V.sub.1 scene and the V.sub.2 scene being in the foreground of the composite scene. However, the conventional mixer does not allow the operator to control on a dynamic basis whether the mixing operation is under the primary control of the signal K.sub.1 or of the signal K.sub.2.